1. Field of the Invention
The present invention is broadly directed to systems for the preservation of organic material by freezing. More particularly, the present invention pertains to systems using fluids for freezing organic materials that have solid or semi-solid components.
2. Discussion of Related Art
Fish and meat products are conventionally frozen by direct contact with chilled surfaces or immersion in brines containing calcium chloride and/or ethylene or propylene glycols, or immersion in the more expensive liquid nitrogen or liquid carbon dioxide. These processes result a in loss of taste and texture in these products, through improper ice crystal formation during freezing. This produces an undesirable concentration of salts in the flesh, as water crystallizes out of the flesh during freezing, and a subsequent loss of naturally-occurring flavored juices that have crystallized with the ice, upon thawing.
Portions of frozen foods that are shipped in heat-sealed plastic bags are well-known in the food industry. The portions in these bags may be scaled for commercial restaurant use as well as for use by individual families or family members. In many instances the food stuffs are vacuum-packed and ready to be heated in the bag, either in a microwave or in boiling water, and served immediately. They are very convenient because they are then ready-to-eat after heating without further preparation.
However, all conventional frozen foods are subject to some structural breakdown, breakdown caused by improper ice crystal formation in the original freezing or improper maintenance of its frozen condition while in storage. This structural breakdown is particularly objectionable in heat-sealed plastic bags containing pre-cooked, ready-to-eat foods. For example: a frozen casserole of cooked, tender stew meat and vegetables rapidly turns to mush, if improperly maintained in the frozen state. Furthermore the result of this is particularly objectionable when it occurs in individual ready-to eat portions. The ice crystals that break up the solid structure of meat and vegetables, when the item partially thaws and then is refrozen, may affect the food uniformly throughout the bag, damaging the entire portion because of the relatively small volume of each portion.
Ready-to-cook meat, vegetable and fish portions, whether raw or pre-treated by superficial searing, broiling or deepfrying are even more sensitive. They are subject to particularly serious quality degradation in the initial freezing process, as well as to the hazards of refreezing, because a greater firmness and definition of texture is expected from filets and steaks, than from pre-cooked, heat-and-eat frozen foods such as stews or sauces.
In the fisheries industry, it was discovered that the addition of cruciferous oils to conventional brines both increased the freezing rate and resulted in an increased thawing rate for fish frozen directly in brine, as discussed in U.S. Pat. Nos. 4,654,217; 4,657,768 and 4,840,035. A rapid freezing rate causes materials to freeze as a block, preventing the growth of a destructive multiplicity of ice crystals that macerates the texture of vegetable foodstuffs as well as flesh. The formation of multiple ice crystals extending radially from the center of a solid item is also believed by some to accelerate drip-related flavor losses during thawing. The rapid thawing rate exhibited by foods frozen in such brines may also prevent a subsequent damage by preventing the formation of new, potentially destructive ice-crystal structures during the intervening half-frozen/half-thawed state.
The operating range mentioned in disclosures of freezing methods using cruciferous-oil brines, -22.degree. and -48.degree. F. (-30.degree. and -44.4.degree. C.), represents the range within which various cruciferous-oil brine formulations can be maintained in the semi-liquid "slush" state. Above that range the brine lacks the necessary ice-crystal content and below that range the brines begin to become nearly solid, too stiff to transfer heat efficiently. However, we have observed that the taste as well as the texture of flesh and other organic tissues is best preserved when the brine cooling it is maintained within a rather narrow range, a range about 4.degree. F. (2.2.degree. C.) wide, during the entire freezing process. The proper set point for this narrow operating range depends on the flow rate of the brine, the volume and cross-sectional dimensions of the item and the quantity of heat stored in each, among other things. A too-fast flow or a too-cold brine is wasteful.
Control of brine-flow dynamics, as well as control of brine cooling and the temperature variations within the bath 6 that are produced by the tendency to thermal layering in laminar brine flows are critically important to maintaining the brine at the surface of the food being frozen within this narrow range to assure that the desired type of ice-crystal formation takes place. For example, 1-inch (2.54-cm) thick salmon steaks should be frozen in a cruciferous-oil brine flowing at 3 feet/minute (0.91 meters) and maintained between -37.degree. and -41.degree. F. (-38.33.degree. and -40.56.degree. C.). In contrast, 2-inch (5.08-cm) thick tuna steaks are oilier as well as thicker, so that the brine needs to be a bit colder, between -38.degree. and -42.degree. F. (-38.9.degree. and -41.1.degree. C.), but need flow only about half as fast through the bath, 1.5 to 2 feet/minute (0.46 to 0.61 meters/minute)
Basically, the interior of each block of tissue must pass must very rapidly through the -0.5.degree. to -5.0.degree. C. range where the damaging ice crystals are most likely to form. The cruciferous oil brines are advantageous for this purpose because, between -33.degree. and -46.degree. F. (-36.11.degree. and -43.33.degree. C.), they produce a conveniently fluid slush that has a relatively high specific heat, so that they are able to transfer more heat per unit volume than other brines used for this purpose. The high specific heat of this slush, representing a negative quantity of heat stored up during the formation of the slush ice crystals in the brine, allows these brines to resist the "shock" effect produced by the initial immersion of large quantities of product in the brine, which results in a localized heating that temporarily "exhausts" of the brine's ability to chill products.
On the other hand, it is also important to avoid the deterioration of cellular structure that occurs when tissues are kept at an excessively low temperature. The cross-linking of molecules and loss of hydration that occurs in the frozen state, problems often referred to collectively as "freezer burn", must be minimized. Because this also tends to limit how low the temperature of the brine can be reduced to prevent brine-shock and brine-exhaustion, it further emphasizes the importance of the high specific heat of cruciferous-oil brines.
In immersion batch-freezing systems conventionally used to freeze whole fish and cuts of raw meat, as well as vacuum-sealed ready-to-eat food portions, ice-crystal formation is poorly controlled. Individual fish or chops, or individual food bags 2, are stacked into freezer bins or piled into an ordinary basket 4 and dunked into a tank 5 holding a liquid nitrogen or brine bath 6, as shown in FIG. 1a. However, in these piles, the inner items freeze slower and, even in single-layer stacks, all items 2 freeze more slowly than they would if they were less closely packed. Furthermore, items in a pile or stack tend to freeze together, requiring separation steps that may either damage the bag or partially defrost the product, risking further damage to its texture and flavor.
The batch-processing basket 4, shown in FIG. 1, is conventionally 3.5-feet high, 3.5-feet thick and 4-feet wide (1.07 m.times.1.07 m.times.1.22 m). Adding shelves to the baskets 4 so as to lay out one layer on each shelf, provides space for a total of 288 lbs (130.64 kg.) of 12-ounce (34-gram) portions of food. Single layers provide a better-controlled rate of freezing by allowing the brine in the bath 6 to flow both over and under each item in the basket 4. However, these portions must still be loaded, unloaded and then rinsed and dried individually. This is labor-intensive and risks damage to the integrity of the bag and to the condition of the food item itself. Also, the shelf-type separators are awkward to carry during loading and unloading, but unfrozen portions may be difficult to load into slot-type separators and ice may form between the slot and the portion during the unloading process, if the ambient air is humid, sticking the portion to the slot.
Conventional brine freezing operations dump the basket 4 of frozen items into a rinse sink, to remove brine residues,and then pulls the items out one-by-one to drain and dry before packaging. In each batch, all the handling required for these processes must be completed very quickly, or under refrigeration, to prevent thawing and consequential recrystallisation. The greater amount of handling required to process small individual portions merely adds to the need for speed. Moreover, the rinse and drying time of each item must be strictly controlled, which is impossible when using a rinse sink.
Fluid-convection and surface-conduction continuous-conveyor freezing systems are used for longer runs of frozen products. U.S. Pat. No. 5,522,227 discloses conveyor-belt system on which each item is first carried into a freezing brine and then drained as the item emerges from the brine. U.S. Pat. No. 4,531,373 discloses a contact-cooling belt that eliminates the draining step. U.S. Pat. No. 4,555,914, and the '183 patent use dual-conduction in contacts between the moving items and two surfaces cooled by brine, brine contained in both a lower steel tank and an upper flexible plastic tank and each tank provides one of the contact surfaces.
In either batch or continuous processes, whenever brines are used, brine residues and brine drip are both problematic. The continuous processes in U.S. Pat. Nos. 4,534,183 the ('183 patent) and 5,168,712 addressed this drip and residue problem by using a steel or plastic web interposed between the brines and material being frozen and then merely separating the web from the frozen item to avoid inconvenient, time-consuming rinsing 8a and drying 8b operations, used in the batch process shown in FIGS. 1a and 1b.